The present invention relates in general to cathode ray tube (CRT) structural arrangements. More specifically, the invention is directed to the structure of a resistor built into the CRT envelope.
In a color CRT a focusing potential of 5 KV to 8 KV is required. A high potential slightly lower than the anode potential, such as 25 KV to 30 KV, is also required in a mask focusing color CRT. Such focusing or high potential voltage are provided to respective electrodes through stem pins mounted on a neck of the tube envelope. To reduce deflection power consumption a diameter of a neck of the tube envelope is narrowed. With the neck narrowed, the pins of the CRT are close to one another. The use of high potential voltages causes the problem of withstanding high voltage at the stem portion of the CRT and also complicates the stem structure.
To solve this problem, a CRT having a resistor built in the tube envelope was proposed in Japanese Utility Model Nos. 48-21561 and 55-38484 and U.S. Pat. Nos. 3,932,786 and 4,143,298. The teachings of these documents are hereby incorporated by reference as if fully set forth herein.
The resistor divides the applied potential, such as an anode potential applied from an anode button provided on a funnel of the tube envelope, into an intermediate high potential such as a focusing potential. The resistor built in the neck of the tube envelope must be made small because of the small space available the neck. The resistor must have a very high resistance value, for example 500.times.10.sup.6 ohms, because a high potential of 25 KV to 30 KV is applied to the resistor and a power consumption in the resistor must be low. Thus, the resistor must have a small volume and a high resistance value. However, it is difficult to control a value of the resistance of such resistor in a mass production.
A known resistor of such type comprises a ceramic insulating substrate and a zigzag shaped resistive layer printed on the substrate. The resistive layer is made of a mixture of ruthenium oxide and glass. However in production, such resistors have resistance values in a range of about .+-.10% depending upon the production process. So, a divided potential provided by such resistors also varies from one resistor to another. Consequently a cathode ray tube having a built-in resistor must have some voltage adjusting means, such as a variable resistor, provided outside of the tube envelope. Even if the adjusting means is utilized, when the deviation from nominal value of the resistance value is large its deviation can not be fully adjusted or compensated by the adjusting means. Further, the built-in resistor set outside of the tube must also have very high resistive value, for example 100 to 500.times.10.sup.6 ohm. Mass production of such high resistance variable resistors is difficult.